Introduction to Programming with MPI
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Practical Exercise 07 (Communicators etc.)
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For general instructions, see the introduction to the collective
practicals.



Question 1
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Take a copy of the program you wrote in question 4.2 in practical
exercise 3, that was called 'Queequeg'.


1.1 Increase the number of processes to 8, and run the program.


1.2 Make sure that the target array of the reduction is cleared to
zero before the reduction, and split the communicator to put the
following processes in new communicators:

    0    not in a new communicator
    1    in new communicator A
    2    in new communicator B
    3    not in a new communicator
    4    in new communicator A
    5    in new communicator A
    6    in new communicator B
    7    not in a new communicator

Rerun the program with the reduction operating in the new communicators,
but everything else operating in MPI_COMM_WORLD.  The value of 'root'
should be left at 1, and note the processes on which the reductions end
up.